1. Field of the Invention
The subject invention relates generally to lasers and, more particularly, to improved preionization apparatus for pulsed gas lasers.
2. Description of Related Art
CO.sub.2 transversely excited atmospheric (TEA) lasers are important components in various range finders, designators, and pulsed radar systems. Reliable designs for such lasers are essential. The discharge preionizer is a central component of such lasers and is a major determinant of overall cost.
High pressure lasers such as the CO.sub.2 type generally employ a gain medium, defined by an opposed rectangular anode and cathode of large aspect ratio The gain medium is excited by pulsed discharges, the pulses being on the order of 100 nanoseconds or less in duration. In order to achieve a uniform discharge throughout the gain medium, uniform preionization is required. The accepted means of achieving uniform preionization in compact tactical lasers is through use of auxiliary corona discharge. The corona discharge creates ultraviolet (uv) radiation which ionizes the gain medium.
In the prior art, corona discharges have been obtained by placing a third grounded and insulated auxiliary electrode adjacent the pulsed main discharge electrode. The auxiliary electrode typically has employed ceramic insulation, which serves as a distributed capacitance. Accordingly, the corona strength is proportional to the power required to charge this distributed capacitance. To achieve a large capacitance and, hence, a strong corona discharge, it is important to have high dielectric field strengths and intimate contact between the ceramic auxiliary electrode and the main electrode.
In order to achieve high dielectric field strength, prior preionizers have used small diameter (.apprxeq.1.6 mm) rods encased in small bore ceramic tubes. These designs, however, are fragile and cannot be made longer than about 15 centimeters (cm). Most importantly, because of lack of structural rigidity, alignment with the main electrode is very difficult to achieve. Approaches using sharp edged electrodes are not producible and are prone to dielectric breakdown because of intense fields.
In one alternative prior art design, the trigger preionization electrode has been configured as a blade inserted into a slot in the encapsulating dielectric. This approach is prohibitively expensive and difficult to produce.
Thus, prior art preionizer approaches have suffered from (1) difficulties in obtaining the required critical intimate contact between the preionizer insulator and main electrode, (2) high production costs, (3) the relatively short electrode lengths that can be machined with the required tolerances, and (4) high voltage breakdown of the insulator.